Ames Laboratory researchers solve fuel-cell membrane structure conundrum

December 11, 2007

AMES, Iowa - Fuel-cell cars are reaching commercial viability in today's increasingly eco-conscious society, but despite their promise, even scientists have struggled to explain just how the fuel-cell's central component - the proton exchange membrane - really works.

However, a team of researchers at the U.S. Department of Energy's Ames Laboratory has offered a new model that provides the best explanation to date for the membrane's structure and how it functions. And armed with that information, scientists should be able to build similar fuel-cell membrane materials that are less expensive or have different properties, such as higher operating temperatures.

A fuel cell works by pumping hydrogen gas through the proton exchange membrane. In the process, the hydrogen gives up electrons in the form of electricity, then combines with oxygen gas to form water as the by-product. It can also work in reverse - when current is applied, water is split into its component gases, hydrogen and oxygen.

The model proposed by Ames Laboratory scientists Klaus Schmidt-Rohr and Qiang Chen, and detailed in the Dec. 9 issue of the journal Nature Materials, looked specifically at Nafion®, a widely used perfluorinated polymer film that stands out for its high selective permeability to water and protons. Schmidt-Rohr, who is also a professor of chemistry at Iowa State University, suggests that Nafion® has a closely packed network of nanoscale cylindrical water channels running in parallel through the material.

"From nuclear magnetic resonance (NMR), we know that Nafion® molecules have a rigid backbone structure with hair-like 'defects' along the chain," Schmidt-Rohr said, "but we didn't know just how these molecule were arranged. Some have proposed spheroidal water clusters, others a web-like network of water channels."

"Our theory is that these hydrophobic (water-hating) backbone structures cluster together," he continued, "to form long rigid cylinders about 2.5 nanometers in diameter with the hydrophilic 'hairs' to the inside of the water-filled tubes."

Though the cylinders in different parts of the sample may not align perfectly, they do connect to create water channels passing through the membrane material, which can be 10's of microns thick. It's this structure of relatively wide diameter channels, densely packed and running mostly parallel through the material that helps explain how water and protons can so easily diffuse through Nafion®, "almost as easily as water passing through water" Schmidt-Rohr said.

To unlock the structure mystery, Schmidt-Rohr turned to mathematical modeling of small-angle X-ray and neutron scattering, or SAXS/SANS. X-ray or neutron radiation is scattered by the sample and the resulting scattering pattern is analyzed to provide information about the size, shape and orientation of the components of the sample on the nanometer scale.

Using an algorithm known as multidimensional Fourier transformation, Schmidt-Rohr was able to show that his model of long, densely packed channels closely matches the known scattering data of Nafion®. Mathematical modeling of other proposed structures, in which the water clusters have other shapes or connectivities, did not match the measured scattering curves.

"Our model also helps explain how conductivity continues even well below the freezing point of water," Schmidt-Rohr said. "While water would freeze in the larger channels, it would continue to diffuse in the smaller-diameter pores."

Schmidt-Rohr added that additional analysis is needed to determine how the cylinders connect through the membrane.
-end-
The research is funded by the Department of Energy's Office of Basic Energy Sciences and conducted by Ames Laboratory's Materials Chemistry and Biomolecular Material Program.

Ames Laboratory, celebrating its 60th anniversary in 2007, is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including the synthesis and study of new materials, energy resources, high-speed computer design, and environmental cleanup and restoration.

DOE/Ames Laboratory

Related Hydrogen Articles from Brightsurf:

Solar hydrogen: let's consider the stability of photoelectrodes
As part of an international collaboration, a team at the HZB has examined the corrosion processes of high-quality BiVO4 photoelectrodes using different state-of-the-art characterisation methods.

Hydrogen vehicles might soon become the global norm
Roughly one billion cars and trucks zoom about the world's roadways.

Hydrogen economy with mass production of high-purity hydrogen from ammonia
The Korea Institute of Science and Technology (KIST) has made an announcement about the technology to extract high-purity hydrogen from ammonia and generate electric power in conjunction with a fuel cell developed by a team led by Young Suk Jo and Chang Won Yoon from the Center for Hydrogen and Fuel Cell Research.

Superconductivity: It's hydrogen's fault
Last summer, it was discovered that there are promising superconductors in a special class of materials, the so-called nickelates.

Hydrogen energy at the root of life
A team of international researchers in Germany, France and Japan is making progress on answering the question of the origin of life.

Hydrogen alarm for remote hydrogen leak detection
Tomsk Polytechnic University jointly with the University of Chemistry and Technology of Prague proposed new sensors based on widely available optical fiber to ensure accurate detection of hydrogen molecules in the air.

Preparing for the hydrogen economy
In a world first, University of Sydney researchers have found evidence of how hydrogen causes embrittlement of steels.

Hydrogen boride nanosheets: A promising material for hydrogen carrier
Researchers at Tokyo Institute of Technology, University of Tsukuba, and colleagues in Japan report a promising hydrogen carrier in the form of hydrogen boride nanosheets.

World's fastest hydrogen sensor could pave the way for clean hydrogen energy
Hydrogen is a clean and renewable energy carrier that can power vehicles, with water as the only emission.

Chemical hydrogen storage system
Hydrogen is a highly attractive, but also highly explosive energy carrier, which requires safe, lightweight and cheap storage as well as transportation systems.

Read More: Hydrogen News and Hydrogen Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.